1. Field of the Invention
The invention relates to a method for the discontinuous metering of compositions based on organopolysiloxanes, in particular organopolysiloxane compositions crosslinkable at room temperature.
2. Description of the Related Art
Single-component sealing compounds that are storable in the absence of water, and vulcanizing on the ingress of water to form elastomers at room temperature are known. These products are used in great amounts in the building industry, for example. The base for these mixtures are polymers which are terminated by silyl groups and which carry reactive substituents such as OH groups or hydrolyzable groups such as alkoxy groups or acetoxy groups. In addition, these sealing compounds can contain fillers, plasticizers, crosslinkers, catalysts and also additives. Very frequently, highly disperse silicic acids are used as fillers. These are added to the “RTV1” sealing compounds in very small amounts, since in case of larger proportions of the reinforcing fillers, undesirably high tensile stresses are reached even at low strains of the cured material. The production of RTV1 sealing compounds is described, for example, in EP-B1-1 884 541.
Conventionally, the RTV1 sealing compounds, after they have been produced, are packaged in barrels or what are termed fluid bags in which they are first stored. Then they are packaged in ready-to-use packages, such as cartridges, flexible pouches or tubes. Usually, the RTV1 sealing compounds are first expressed from the large-scale package at a certain upstream pressure or removed by suction. Thereafter transporting elements such as screw pumps or gear pumps are used.
The transporting processes are generally technically very demanding, since, in contrast to Newtonian fluids, the firm RTV1 sealing compounds exhibit a completely different rheological behavior. In particular, RTV1 sealing compounds, in addition to their extremely high viscosity, exhibit a flow limit, since they must not flow out of a vertical joint when used, for example, for sealing building joints.
Therefore, at each start of a transporting operation, first the flow limit must be overcome, before the high-viscosity RTV1 sealing compound begins to move in the transporting line.
This means that a considerable amount of power must be introduced into the mixture via the transporting elements. Such a power introduction is highly disadvantageous here, since the high mechanical stress of at least the part of the mixture which is situated in the transporting elements, degrades the structures of the reinforcing fillers. This can even lead to loss of firmness of the RTV1 sealing compound.
The higher the volume that is to be transported, the greater the power input and the mechanical stress of the RTV1 sealing compound in the transporting element at the start of each transporting operation.
Since this mechanical stress in the transporting element relates to only a relatively small part with respect to the total volume that is to be transported and since backmixing processes also take place during the further transporting operation, the product damage can very frequently be tolerated if a relatively long-lasting transporting operation follows.
However, the situation is completely different if the RTV1 sealing compounds are packaged in small packages such as 310 ml cartridges. In this case, the product damage to the RTV1 sealing compound owing to mechanical stress can be dramatic, since the transporting operation is constantly interrupted and must be started again each time for packaging in a cartridge. In particular, the product damage can be very high when the volume of the RTV1 sealing compound in the transporting element is greater than 310 ml. This would mean that some of the sealing compound is actually repeatedly highly mechanically stressed. This is the case, in particular, when, because of intentionally short metering times, metering pumps having a very large metering volume are used.
In another method for metering RTV1 sealing compounds at packaging machines, after metering the desired amount, the metering line is likewise closed. The metering pump, however, remains permanently in operation and transports the RTV1 sealing compound via what is termed an overflow back into the line upstream of the metering pump. The product damage caused thereby is unwanted. However, it is further increased in that, for example, during machine shutdowns, pumped circulation via the overflow is operated over a relatively long period.
In addition, during packaging in small packages, owing to the discontinuous transport, unwanted pressure pulses act on the mechanical equipment of the packaging device. This produces costly leakage losses in the case of hydraulic oils, leaks at transitions from fixed metal tubes to flexible tube lines, a great hazard owing to large gas volumes in pneumatic plants and a high noise level.
In addition, there is the disadvantage that the packaging plants are idle over a relatively long time because the packages from which metering takes place must be changed after they are emptied. These idle times drastically shorten the machine running times.